This invention relates to a test circuit for an indicator lamp and more particularly to an improved circuit for simultaneously testing indicator lamps utilizing a self-triggering bidirectional thyristor.
On modern machinery there are many applications where multiple indicator lights are used. It is advantageous to have a means of rapidly testing many indicator lamps by depressing a single pushbutton. A problem with some of the prior art test circuits is that many relays or multipole pushbuttons are required in the test circuit. In some of the prior art indicator circuits, the normal signal circuits, the test circuits and the indicator lamp circuits are all operated at the same potential. It is desirable to have a circuit in which the test pushbutton and the indicator signal are at a higher normal operating voltage, such as 120 volts, while the indicator lamp is maintained at a low voltage, such as 6 volts, for system safety and cost.
In prior art U.S. Pat. No. 3,040,243 issued June 19, 1962 to I.F. Weiss a test circuit for an indicator system utilizing a single test button is disclosed. A problem with this circuit as disclosed in Weiss is that a separate transformer is required for the test circuit. The separate transformer can cause polarity and installation disabilities. In this prior art circuit the isolating diodes are disposed on the low voltage side of the indicator transformers. In a low voltage indicator lamp circuit voltage drop across the diodes can be significant. Also the disclosed circuit applies 1/2 wave rectified current to the bulb in both the normal and test conditions making it necessary to apply 8 to 10 volts or more to get a bright enough indication.
It is desirable to have a single pushbutton test circuit and operating circuit combination which is highly reliable and not susceptible to showing erroneous indications.